Catechism of the Catholic Church §246 — excerpt
The Latin tradition of the Creed confesses that the Spirit "proceeds from the Father and the Son (filioque)". the Council of Florence in 1438 explains: "The Holy Spirit is eternally from Father and Son; He has his nature and subsistence at once (simul) from the Father and the Son.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: Council of Florence (1439): DS 1300-1301